Devices utilizing the giant magneto-resistance (GMR) effect have utility as magnetic sensors, especially as read sensors in read heads used in magnetic disc storage systems. The GMR effect is observed in thin, electrically conductive multi-layer systems having multiple magnetic layers. One sensor type that utilizes the GMR effect is the GMR multi-layer. The GMR multi-layer typically comprises a series of bi-layer structures, each of which comprises a thin sheet of a ferromagnetic material and a thin sheet of a non-magnetic material. The bi-layers are stacked to form a multi-layer device. The multi-layer device is typically mounted in the read head so that the planes of the films are perpendicular to the surface of the disc. The magnetization of each ferromagnetic layer in the multi-layer device is approximately orthogonal to the magnetization of adjacent ferromagnetic layers and, when used in a magnetic disc storage system, would be oriented in a plane perpendicular to the plane of the disc.
In operation, a sense current flows through the read head and therefore through the sensor. Magnetic flux from the disc causes a rotation of the magnetization vector in at least one of the layers, which in turn causes a change in the overall resistance of the sensor. As the resistance of the sensor changes, the voltage across the sensor changes, thereby producing an output voltage.
The sense current can flow through the sensor in a direction that is parallel to the planes of the layers or stacked strips. This is known as a current-in-plane (CIP) configuration. The output voltage produced by the sensor is affected by various characteristics of the sensor.
One approach to reaching higher data storage areal densities is to use perpendicular magnetic storage media. As written bits in the media become smaller, readback sensors that are more sensitive are needed. A standard readback sensor gives a square wave output when reading from perpendicular media. This output contains a DC component that complicates the channel. One proposed solution is to differentiate the readback signal, but this leads to added noise. A second approach is to use a readback sensor that differentiates the flux from the media. This type of sensor would give a positive or negative voltage spike at a magnetic transition in the media. The orientation of the voltage spike would depend on the orientation of the transition (up-to-down magnetization or down-to-up magnetization).
There is a need for sensors that provide a high output in combination with flux differentiation.